1. Field of the Invention
This invention relates to a silicon pressure sensor chip. More specifically, this invention relates to a silicon pressure sensor chip using a shear element located on a thick portion of the diaphragm.
2. Description of the Prior Art
Silicon pressure sensor chips are well known in the art. Typical prior art sensor chips have a shear element, i.e., a piezo-resistive sensing element, placed on a square diaphragm of constant thickness, or alternatively use a Wheatstone bridge with resistors located in thin flexure areas of a sculptured diaphragm. The use of a sculptured diaphragm for improved sensor performance is described in U.S. Pat. No. 4,065,970 issued Jan. 3, 1978 to Wilner This disclosure describes a diaphragm for a pressure transducer having a surface sculptured by anisotropic etching to provide gauge areas in the form of narrow thin flexure areas between thick areas in the form of islands. Also, U.S. Pat. No. 4,236,137, issued Nov. 25, 1980 to Kurtz, et al. describes (See FIG. 1) a silicon chip 2 (in original) serving as a pressure transducer with a back surface 4 in which is located a central boss 6 (12 in original) of a trapezoidal cross-section surrounded by a groove 8 (11 in original) of a particular width. Piezo-resistive sensing elements (not shown) are formed on the front surface 10 in the ring-shaped diaphragm defined by groove 8.
In both of the above disclosures, anisotropic silicon etching provides stress concentration in thin flexure areas in which the piezo-resistive sensing elements are located. In both cases, the devices are optimized to generate uniform maximal stresses for longitudinal and transverse piezo-resistors connected to a Wheatstone bridge.
In U S. Pat. No. 4,236,137 it was noted that the problems of intrinsic diaphragm stresses due to the thermal mismatch between silicon and thin surface films presents a problem. The stress is noted as being increased if the thickness of the silicon flexures is reduced for purposes of stress concentration.
Therefore the prior art of FIG. 1 typically places the sensor elements on a thin portion of the diaphragm, because the thick portion of the diaphragm does not provide enough sensitivity. This prior art structure therefore disadvantageously increases stresses in the diaphragm from secondary effects, which worsens the problems of voltage offset and hence produces a sensor with significant deficiencies.
FIG. 2 shows a computer-simulated distribution of shear stress for a prior art square diaphragm 14 of constant thickness (thus differing from that shown in FIG. 1) with built-in edges 16, piezo-resistive shear element 18 in the form of a four-point resistor, and contours 20-1, 20-2, . . . 20-k of constant shear stress magnitude shown by dotted lines caused by a pressure difference across the diaphragm 14.
FIG. 2 shows contours by the dotted lines 20-1, 20-2, . . . 20-k for ten different stress levels with constant stress increase. Zero stress is formed along the diagonals 22a, 22b of the diaphragm 14. Increasing stress levels lead to points of maximum stress at the midpoints of the diaphragm edges. Thus, there is shown a four-fold symmetry of the square diaphragm. This symmetry of the structure of FIG. 2 is used in the prior art to place four different shear elements on the diaphragm.